1. Field of the Invention
The present invention relates to a core material for the construction of steering wheels of automobiles which is formed by superposing continuous resin impregnated, long-staple fiber strands with a hardening resin.
2. Description of the Background
Steering wheels of automobiles have in the past been made of synthetic resins in order to help in the reduction of the overall weight of automobiles. However, the synthetic resins alone do not provide a steering wheel with strength. In order to increase the strength of steering wheels, therefore, the core material generally has been made of steel. This fact, however, precludes any substantial weight reduction in manufactured steering wheels.
A method which has been developed in the past which enables the preparation of a predetermined shape by winding a continuous long-staple fiber strand that is obtained by impregnating continuous long-staple fibers (roving or strand) such as carbon fibers with a synthetic resin such as epoxy resin or the like (see British Patent Publication GB No. 2004835A) on a frame or a mold. The structure which is formed by this method exhibits great compressive strength or tensile strength in the lengthwise direction of the fibers. FIGS. 10 and 11 illustrate core mateirals which have been used in the preparation of steering wheels by the above method. FIG. 10 shows a conventional technique in which a continuous fiber strand is superposed from the side of a ring portion 11a of a steering wheel core material 11 to the side of a boss 12 to form a spoke portion 13. The fibre strand is then wound around the periphery of the boss 12, and is returned to the side of the same spoke portion 13 or another spoke portion 13 thereby forming the whole at spoke portions 13.
FIG. 11 shows a conventional method in which the fiber strand from the side of a ring portion 11a of the steering wheel core material 11 is wound on a pin 14 provided on the boss 12, and then the fiber strand is superposed and stretched again toward the side of ring portion 11a, to form the spoke portions 13. The steering wheel core material 11 thus obtained is lighter than conventional materials, because a fiber-reinforced resin is used to form the core material instead of a steel core. However, these methods result in the occurrence of voids S in the base of spoke portions 13 on the side of boss 12, as shown in FIG. 12, where the superposed fiber strand 15 is partially branched. Therefore, the continuous long-staple fiber strand 15 has a reduced density at this branched area, and as a result spoke 13 loses stiffness. Further, if an external force is imparted to the steering wheel core material 11, it is probable that the superposed fibers will peel from one another on the side of the boss 12, because the direction of the force imparted is different from the lengthwise direction of the continuous long-staple fibers constituting the spoke 13. A need therefore continues to exist for an improved method by which such structures as steering wheels can be made of light weight materials, while retaining good strength characteristics.